Rubber articles, such as tires, belts and hoses, normally use reinforcing materials such as steel, polyester, nylon, aramid and rayon in the form of fibers, cords or fabrics. In the case of radial tire production, steel cords are often used as the reinforcing material. Steel cords for tires, in general, are coated with a layer of brass to promote adhesion between the steel cords and rubber compounds. In order to improve the adhesion between rubber and steel cords, the use of a cobalt salt, such as cobalt naphthenate, and/or a phenolic adhesive composition comprising a methylene acceptor and methylene donor in the compound formulations are in current practice. The copper and zinc present in the brass coating react with sulfur to form a bonding layer, comprising sulphides of copper and zinc, between the steel cord and rubber. The formation of such sulfide layers at the interface is responsible for the initial unaged adhesion of brass plated steel and rubber. The use of cobalt salt regulates the formation and composition of copper sulfide and zinc sulfide layers. It has become common to use resorcinol or resorcinolic novolak resins as the phenolic methylene acceptor and hexamethoxymethylmelamine (HMMM) or pentamethoxymethylmelamine (PMMM) as the methylene donor in rubber compounds to improve the steel cord adhesion. On curing, the reaction product of phenolic methylene acceptor and donor form a protective moisture resistant resin coating on the bonding layer and protects the loss of adhesion during aging.
Achieving higher levels of steel cord adhesion with rubber compounds and maintaining this adhesion under various environmental conditions, such as heat, humidity and saline conditions, are important for the long term durability of tires. In the case of an unaged condition, rubber-brass adhesion exceeds the tear strength of the rubber and therefore, no bond failure occurs at the copper sulfide and rubber interface. But, under wet and salt water conditions the adhesion of steel cords fails due to corrosion. Though the cobalt salt was effective against salt water and steam-aged adhesion, the use of resorcinol or resorcinolic novolak resins, along with HMMM, has provided the highest adhesion under these conditions.
The corrosion of steel cords is due to the attack of moisture under hot and wet conditions. If this is prevented, then the corrosion of steel wire can be avoided, thereby the adhesion level can be maintained under all aged conditions of the tire. In this way the service life of the tire can be extended.
In order to avoid or minimize the steel tire cords corrosion, several approaches or methods were employed, not only in the rubber compound formulations but also with the treatment of steel cords.
In one method, steel wires were cleaned first and then coated with an amino-silane primer. Then the silane coated wires were again coated with a phenol-resorcinol-formaldehyde-latex solution before being incorporated into the rubber compound and cured. This method improved the hydrolytic stability of the bond between the steel and rubber.
In another method, the humidity-aged adhesion of brass plated steel cord to rubber was improved by dipping the cord in dilute acetic acid in methanol solution followed by a treatment with H2S gas. The cord was then combined with a vulcanizable rubber compound and cured.
Bright steel wires dipped in the adhesive compositions prepared by the mixing of phenol-formaldehyde resole and resorcinol-formaldehyde novolak solutions showed a dramatic improvement over the brass plated wires in the adhesion retention after humidity aging. The highly cross-linked phenol-resorcinol-formaldehyde network formed from the resole and novolak that coated the bright steel is responsible for the moisture resistance and improving the humidity-aged adhesion.
Improved humidity-aged adhesion between a rubber compound and bright steel was achieved when the bright steel wires were dipped first into an alcoholic solution of aminosilanes and then vulcanized with a rubber containing a phenolic novolak resin. Compared to brass-plated steel cords, the silane-treated bright steel wires retained their high adhesion values after humidity aging.
The application of silanes and hydrolyzed silanes on the surface of reinforcing tire cords such as stainless steel, galvanized steel, tin, zinc or brass plated steel are known to prevent the corrosion of these metals. On hydrolysis, these silanes produce silanol groups which are active towards the hydroxyl or oxide groups present on the surface of these metals. The silanol groups react themselves producing Si—O—Si bonds on the metal surface and are more stable and hydrophobic. This makes the silane-treated metal surfaces more resistant to moisture and corrosive attack. Though this method provides a solution to prevent the corrosion of steel cords, the use of highly flammable solvents to dissolve these silanes, their applications onto these metal surfaces, and handling thereof can be an environmental hazard.
Therefore, there is a need for a new phenolic resin that can be handled and used relatively safely in rubber compound formulations and, in the meantime, improves the unaged, heat-, and humidity-aged adhesion of a brass-plated steel cords to cured rubber compounds.